The invention relates to apparatus for locating objects behind wall linings, and in particular wall framing structures such as studs. Such apparatus are commonly known as stud detectors or stud sensors.
Although the expression “stud” is used, such being a structural member used in building construction typically for supporting internal walls, the expression “stud sensor device” is to be interpreted broadly as indicating a device for locating a variety of objects of similar form including joists and the like.
A variety of such stud sensors are known, generally being configured to detect changes in dielectric constant. The sensor incorporates a capacitor plate mounted in the sensor and a circuit for measuring change in capacitance. The sensor is moved over the wall surface, the variation in dielectric constant being indicated as a change in capacitive effect which is detected. One such sensor is described in U.S. Pat. No. 4,464,622 (Franklin).
To the user it is desirable to be able to accurately detect the stud position, notably its edges and preferably its centre. It is also desirable to be able to detect the depth of the stud beneath the wall covering or lining, this depth also influencing the accuracy of detection of stud position. Typically, in the US wall coverings have thickness graduated in inches from ½, ⅝, ¾, 1 5/4, 1½, 2 inches.
U.S. Pat. No. 5,352,974 discloses a stud sensor of dual sensitivity, the user being able to manually select sensitivity based on the sensor response.
Applicant's U.S. Pat. No. 6,894,508, the contents of which are hereby incorporated by reference, discloses a stud sensor for detecting the edges of objects behind a wall lining including an input device for allowing a user to select from multiple sensitivities.
Applicant's US 2005/0138886 discloses a stud sensor which includes multiple sensitivity, whereby a user can manually select a required sensitivity according to wall lining thickness. On start up of the unit a reference sensitivity is set associated with the usual minimum wall thickness of ½ inch. The user is then able to manually increase the sensitivity in predefined steps.
In a first aspect the present invention is directed to providing a sensor able to accurately detect the centre of an object behind a wall lining.
According to a first aspect of the invention there is provided a sensing device having a sensing surface for placing against a wall for detecting objects behind a wall lining comprising a first sensor comprising at least first and second capacitor plates and associated resistors forming first and second RC circuits of time constants dependent on the adjacent wall material, a first detection circuit connected to the first and second RC circuits and providing a first voltage signal of amplitude dependent on a change in the time constants of first and second RC circuits, a second sensor comprising at least third and fourth capacitor plates and associated resistors forming third and fourth RC circuits, a second detection circuit connected to the third and fourth RC circuits and providing a second voltage signal of amplitude dependent on a change in time constants, a comparison means configured to receive said first and second voltage signals, and compare them to establish the magnitude of the voltage difference between said voltage signals, and to indicate a detected object centre position when said difference signal is less than or equal a predetermined value.
The first detection circuit preferably includes respective monostable devices triggered by said first and second RC circuits, outputting digital signals of period dependent on the time constant of the RC circuits.
The second detection circuit preferably includes a logic gate receiving said digital signals and outputting said first voltage signal of amplitude dependent on the change of the time constants.
The first and second sensors each comprise a primary capacitor plate, and a secondary capacitor plate of smaller area than said primary plates. This sensor structure is able to provide a particularly sharp response.
The comparison means is preferably incorporated into a processor in which is stored the predetermined value, which may be about 0.04V.
The processor may be configured to detect said voltage signals and to effect a voltage difference calculation when either of the voltage signals exceeds a threshold associated with a background level. Moreover it may be configured to provide a calibration dc bias voltage to said RC circuits in order to maintain a voltage signal of a predetermined magnitude, which calibration may be effected simultaneously for both the circuits.
In a further aspect the invention provides a method of detecting objects beneath wall linings using a sensing device having a first sensor comprising at least first and second capacitor plates and associated resistors forming first and second RC circuits, and a first detection circuit providing a first voltage signal of amplitude dependent on a change in time constants of said RC circuits, a second sensor comprising at least third and fourth capacitor plates and associated resistors forming third and fourth RC circuits, and a second detection circuit providing a second voltage signal of amplitude dependent on a change in time constants of said RC circuits, the method involving the steps of repeatedly detecting said first and second voltages as the sensor is moved over the wall surface, calculating a difference signal, and indicating an object centre position when said difference signal is below a predetermined value.